cot-desc: optimise memory further

This changes the auth_img_desc_t struct to have pointers to struct
arrays instead of struct arrays. This saves memory as many of these
were never used, and can be NULL pointers. Note the memory savings are
only when these arrays are not initialised, as it is assumed these
arrays are fixed length. A possible future optimisation could allow for
variable length.

memory diff:
bl1:        bl2:
    text        text
      -12         -12
    bss         bss
      -1463       0
    data        data
      -56         -48
    rodata      rodata
      -5688       -2592
    total       total
      -7419       -2652

Change-Id: I8f9bdedf75048b8867f40c56381e3a6dc6402bcc
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>